1. Field of the Invention
The present invention relates to an apparatus and a method for manufacturing a fiber reinforced thermoplastic resin sheet. More particularly, the present invention relates to an apparatus for manufacturing a fiber reinforced thermoplastic resin sheet, which can uniformly distribute solid concentration without being affected by inertia forces, and can permit inhibition of variations of unit weight distribution of a web under the effect of the dynamic pressure of the raw material dispersing liquid and the resultant fluctuations. This invention also relates to a method for manufacturing the novel fiber reinforced thermoplastic resin sheet.
2. Description of the Related Art
Attempts have been made to impart high strength and a high toughness value to a thermoplastic resin while retaining processing advantages. Composite material technology has been applied, based on addition to a resin of a fiber having a high modulus of elasticity. Resultant fiber reinforced thermoplastic composite materials have been employed as materials for various structural members required to have light weight, a high toughness value and a high value of strength. These materials are usually heated to above the melting point of the thermoplastic resin, followed by addition of the matrix, and the resulting composite is then formed into a prescribed shape.
Among other composites, plate-shaped or sheet-shaped materials suitable for forming on a press or for forming a large-sized part are known as stampable sheets. Stampable sheets are widely used in automotive structural parts such as seat backs, rear packages and integrally formed automobile ceilings. They achieve a lighter weight and reduce cost. They often reduce the number of necessary parts and of assembly processes. Composite technology is further expanding to include other parts than those mentioned above.
A so-called wet manufacturing method using paper-making technology is known as a typical manufacturing method of making a stampable sheet. This method comprises dispersing fiber chips and a thermoplastic resin in an aqueous solution (dispersing step), preparing a nonwoven fabric-like web by use of paper-making technology by dispersing liquid on a mesh belt (web forming step), and solidifying the web achieved after applying heat and pressure (sheet making step), thereby manufacturing a stampable sheet. A continuous manufacturing method is disclosed in, for example, Japanese Unexamined Patent Publication No. 6-158227.
A web of a stampable sheet based on the paper-making method can be continuously manufactured as follows by the use of a manufacturing apparatus as shown in FIG. 1 of the drawings.
The apparatus shown in FIG. 1 broadly comprises a material preparing section 1, a web-forming section 10, a drying section 40, and a take-up section 50.
A dispersion tank 3 is provided with a stirrer 2 in the material preparing section 1, and a resin feeder 4 feeds stored thermoplastic resin while a reinforcing fiber feeder 5 feeds stored reinforcing fibers from above the dispersion tank 3.
The thermoplastic resin of the resin feeder 4 and the reinforcing fiber of the reinforcing fiber feeder 5 are charged at a prescribed ratio into the dispersion tank 3, which is filled with an aqueous solution containing a surfactant or a viscosity enhancing agent and stirred to prepare a dispersing liquid C serving as a raw material liquid. The thus prepared dispersing liquid C is sucked out of tank 3 by a metering pump 6, and is distributed by a distributor 20 called a manifold into a plurality of branch pipes 9, and then conducted to the web-making section 10. (FIG. 1 shows only one branch pipe 9 extending to inlet 30 for ease of understanding.)
The web-forming section 10 includes at least the following components: a continuously moving endless mesh belt 11, a suction box 12 arranged in contact with the back thereof, and a headbox 13 for storing the dispersing liquid C sent from the material preparing section 1 and supplying the same onto the mesh belt 11. The dispersing liquid C is sent through an inlet 30 to the headbox. The mesh openings of the mesh belt 11 are smaller than the diameter of the reinforcing fiber and the thermoplastic resin. The dispersing liquid C is sucked by the suction box 12 under reduced pressure, and the reinforcing fiber and the thermoplastic resin are converted into a sheet shape (filtered) onto the mesh belt having mesh openings smaller than the particle size of the thermoplastic resin. The mixture in the form of a nonwoven fabric of the reinforcing fiber and the thermoplastic resin thus made into sheets is called a web. The web W, being in a wet state at this point, is passed through a drying section 40.
In the present invention, an inlet and a headbox are arranged very close to each other and they are sometimes referred to as a "distributor".
The drying section 40 has a belt conveyor 41 and a drying chamber 42 connected together downstream of the mesh belt 11, and continuously dries the web W. During the drying process, the resin is melted by removing moisture and changing the temperature of the resin to at least the melting point of the thermoplastic resin to strengthen intertwinement of the reinforcing fiber. The resultant dried web has a high fracture resistance and is excellent in form stability, and is wound to a roll shape on a take-up roll 51 of a take-up section 50.
It is possible, as desired, to impregnate fibers with the thermoplastic resin by cutting and then heat-pressing the web. The product in this state is called a "consolidated sheet." This consolidated sheet is generally used as a forming material.
The quality and properties of a stampable sheet based on the paper-making method are mostly determined during the web-forming step. Also, keeping a uniform unit weight distribution in the web is particularly important in order to reduce any differences in thickness or in the content of the reinforcing fiber and to reduce fluctuations in properties.
Patterns of unit weight distribution are broadly classified into those occurring in the longitudinal direction of the manufacturing line (hereinafter sometimes referred to as the "line direction") and those occurring in the width direction. The former are often attributable to pulsations in the raw material supply system typically represented by a pump, and techniques to prevent this are known. In order to avoid a nonuniform unit weight distribution in the width direction, it is necessary to uniformly expand the raw material (liquid) sent usually through a single pipe. This problem could not, however, be easily solved in the wet manufacturing method of stampable sheets as described later.
Furthermore, the unit weight distribution in the width direction exerts an important effect also on the orientation of the reinforcing fiber for the following reason. When a change occurs in the unit weight distribution in the width direction, the raw material dispersing liquid supplied to the headbox naturally flows into a portion with a smaller unit weight value and a smaller suction resistance, and this causes production of a channelling effect. Because fibers are oriented along the direction of flow, channelling takes place and leads to local abnormal orientation of the fibers, and eventually results in a camber, arching, curvature or the like of the stampable sheet.
In other words, achievement of a uniform unit weight distribution in the width direction is a prerequisite for obtaining sheets having uniform properties.
A uniform unit weight distribution in the width direction can be achieved by using uniform chemical compositions of solids (reinforcing fiber and thermoplastic resin) in the dispersing liquid and supplying the dispersing liquid onto the sheet-making surface with a flow rate that is uniform in the width direction. The manifold and the inlet serve this function.
Problems involved with the manifold and the inlet in currently used wet-type apparatus and solutions to those problems according to the present invention will be described below.
Conventional manifold
The function of the manifold 20 (as shown in FIG. 1) is to supply the dispersing liquid at a flow rate and a solid concentration that is essentially uniform in the width direction of the inlet 30. That is, the manifold 20 serves to widen the flow of the dispersing liquid directed toward the headbox 13 from the dispersing tank 3 through the piping, in the width direction.
The shapes of various manifolds have conventionally been studied from various points of view. In the paper industry, which faces its own specific problems, it is the general practice to use a taper-type manifold of which the cross-sectional area gradually decreases as disclosed in the Journal of the Japan Society of Paper/Pulp technology (published on Jan. 1, 1993), Vol. 47, No. 1, p. 102. An outline of this manifold is illustrated in FIG. 9. According to the literature, the flow rate distribution in the width direction of the headbox 13 is very uniform. Solids are almost exclusively pulp (wood fiber) and there is no problem in the uniform distribution of its concentration.
The foregoing taper-type manifold has been used with very different technology in conventional wet manufacturing apparatus of fiber reinforced thermoplastic resin sheets. However, when using a taper-type manifold for making of stampable sheet, the concentration distribution of solids was not necessarily accomplished uniformly as in the pulp concentration.
The reinforcing fiber and the thermoplastic resin composing a stampable sheet usually have different values of specific gravity and geometric shapes. Typical reinforcing fibers include glass fiber, carbon fiber, and stainless steel fiber. In the case of glass fiber, for example, those having a specific gravity of about 2.54 and an aspect ratio (fiber length/fiber diameter) within a range of from about 1.times.10.sup.6 to 5.times.10.sup.6 are used.
Thermoplastic resins serving as the matrix have, on the other hand, a specific gravity within a range of from about 0.9 to 1.5. For the purpose of reducing the manufacturing cost, it is the usual practice to use a granular shape having a size of about 1 mm, the aspect ratio being about 2 as a maximum.
As shown in FIG. 9, the conventional taper-type manifold turns the direction of the dispersing liquid flowing into it through the inlet port at a right angle, and sends the liquid through the branch pipes 9 to the headbox 13. At this point, the solids in the liquid are under the effect of an inertia force depending upon the shape thereof (aspect ratio). More specifically, because the thermoplastic resin has an aspect ratio that is far smaller than that of a reinforcing fiber (resulting in a larger inertia), the turning of direction near the discharge exit to the branched pipes 9 becomes insufficient and the resin tends to be thrown toward the circulation flow exit Jo. As a result, the thermoplastic resin segregates near the tip of the taper-type manifold. In contrast the reinforcing fibers that have a large aspect ratio (resulting in a smaller inertia) easily follow the flow of the dispersing liquid, causing no segregation. Accordingly, non-uniform concentrations of solids occur in the interior of the manifold because of the inertia differences.
In the conventional wet manufacturing apparatus of stampable sheets, this non-uniform concentration causes a non-uniform unit weight distribution in the width direction of the web, and this in turn causes non-uniformity of mechanical properties such as bending strength and bending elastic modulus of the product sheet, and further causes variations of quality such as non-uniform thicknesses.
Conventional inlet
As shown in FIG. 1, the dispersing liquid distributed at the manifold 20 into the branched pipes 9 is directed to the inlet 30, where the flows meet, and is sent to the mesh belt 11 as a two-dimensional flow. In other words, the inlet 30 may be considered an apparatus which converts the dispersing liquid sent in the form of a plurality of "lines" from the manifold 20 into a "plane" flow and supplies it to the mesh belt 11.
In the conventional wet manufacturing apparatus of stampable sheets, it is the common practice to use a larger-volume inlet 30 to cause the dispersing liquid C to display fully its mixing and straightening effects, with a view to making a web having a uniform unit weight distribution.
However, with an inappropriate increase in volume at the inlet 30, it is impossible to control fluctuations in the flow leaving the exit, and the problem of promoting fluctuation is posed. The fluctuation as referred to herein is a phenomenon in which the flow does not run straight from the exit in the line direction, but horizontally deviates right and left. Occurrence of this fluctuation is attributable mainly to variations of dynamic pressure and flow rate of the dispersing liquid C.
Another problem is one that is intrinsic in the wet manufacturing process. In a wet manufacturing method in which the reinforcing fiber and the thermoplastic resin are transported by a foamed liquid, and wherein there exists the presence of a portion at which the flow velocity decreases, the liquid often separates into a portion containing a large foam content (a high air content) and into another portion containing the aqueous solution without substantial foam. This foam reduces the solid transporting capability of the dispersing liquid and makes it very difficult to achieve a uniform concentration and a uniform unit weight distribution. This decrease in flow velocity of the dispersing liquid tends to occur particularly when the inlet has a structure in which stagnation of the dispersing liquid occurs.
In the wet manufacturing method, therefore, a very difficult task is presented that requires inhibiting the large dynamic pressure of the dispersing liquid and straightening the flow, all without reduction of flow velocity.